1. Field of the Invention
The present invention relates to a microphone unit suitable for a highly directional microphone, the microphone unit allowing—vibration noise and wind noise to be reduced and exhibiting enhanced acoustic performance. The present invention also relates to a highly directional microphone including the microphone unit.
2. Related Background Art
A highly directional microphone includes a microphone unit incorporated into a long and thin acoustic tube. Wind blowing at the open end of the acoustic tube causes a pressure difference between the front and rear of a diaphragm included in the microphone unit. Such a difference causes wind noise. The highly directional microphone has a long distance between a front acoustic terminal, which is provided in the front end of the acoustic tube as a sound wave inlet, and a rear acoustic terminal, which is provided in the rear end of the acoustic tube. Accordingly, a pressure difference can be readily generated between the front acoustic terminal and the rear acoustic terminal. With the diaphragm of the microphone unit provided between the front acoustic terminal and the rear acoustic terminal, the pressures difference is generated between the front and rear of the diaphragm, thus resulting in large wind noise in a low frequency range in particular.
The air inside the acoustic tube acts as an acoustic mass in the low frequency range. Application of the acoustic mass to the diaphragm increases vibration noise in the low frequency range. The wind noise and the vibration noise mainly include low frequency components. In order to reduce noise output, a low-cut filter may be used in an electronic circuit of a highly directional microphone. Since a displacement of the diaphragm due to wind or vibration, however, modulates audio signals, use of the low-cut filter results in an insufficient noise reduction effect.
To address the circumstances, a highly directional microphone is proposed which includes a microphone unit provided with a gap from an internal peripheral surface of an acoustic tube so as to form a connecting path between a front acoustic terminal and a rear acoustic terminal, as is disclosed in, for example, Japanese Unexamined Patent Application Publication No. S62-118698. Such a highly directional microphone can reduce wind noise regardless of a pressure difference generated between a front acoustic terminal and a rear acoustic terminal, since air on a high pressure side flows into a low pressure side through such a connecting path to balance the pressure difference.
In the microphone disclosed in Japanese Unexamined Patent Application Publication No. S62-118698, however, a microphone unit is positioned with a gap from an internal peripheral surface of an acoustic tube. This configuration causes an inevitable increase in the size of the acoustic tube and thus an increase in the size of the entire device. In the case of a small highly directional microphone including an acoustic tube with an external diameter of 8 mm, for instance, the ratio of the external diameter of a microphone unit to the internal diameter of an acoustic tube is significantly small. In the case of a condenser microphone unit, the effective capacitance is reduced in proportion to a reduction in size. Thus, the sensitivity and S/N ratio are low with such a highly directional microphone having the acoustic tube having a small external diameter.
Japanese Unexamined Patent Application Publication No. H4-246999 discloses a highly directional microphone, as shown in FIG. 6, in which a connecting path C′ connecting a front acoustic terminal 2 and a rear acoustic terminal 3 is provided in a unit casing 4 of a microphone unit 40. In the highly directional microphone shown in FIG. 6, even if a pressure difference is generated between the front acoustic terminal 2 and the rear acoustic terminal 3, air on a high pressure side flows into a low pressure side and thus the pressure difference is balanced.
Thus, the highly directional microphone shown in FIG. 6 can reduce wind noise generated by the pressure difference. Furthermore, the connecting path C′ provided in the interior of the unit casing 4 can eliminate necessity of a gap to be formed between the internal peripheral surface of the acoustic tube and the external peripheral surface of the microphone unit. Accordingly, the diameter of the acoustic tube can be reduced compared to the case of the highly directional microphone disclosed in Japanese Unexamined Patent Application Publication No. S62-118698.
If provided with an acoustic tube having an entire length of 100 mm or more, for example, in order to ensure high directivity in a low frequency range, however, the highly directional microphone shown in FIG. 6 does not have a sufficient noise reduction effect unless the acoustic impedance of the connecting path C′ having a width B′ is lowered in the unit casing 4. A large width B′ of the connecting path C′ increases the eccentricity of built-in components 5 other than the unit casing 4 in the microphone unit 40 relative to the internal diameter of the unit casing 4. The axis A of the microphone unit 1 may then be moved to a position A′, for example, thus adversely affecting the acoustic performance of the microphone unit 40.
In addition, a metal mesh 50 is disposed between the unit casing 4 and a diaphragm holder 14 to ensure the connecting path C′ to the front acoustic terminal 2 in the case of the highly directional microphone shown in FIG. 6. The metal mesh 50 itself has an air gap, which causes a gap between the unit casing 4 and the diaphragm holder 14. The connecting path C′ is thus open in the anteroposterior direction of the microphone unit. In a portion composed of the metal mesh 50 in the connecting path C′, however, the metal mesh 50 is compressed and thinned by the unit casing 4 and the diaphragm holder 14. The metal mesh 50 thus serves as acoustic resistance, causing an adverse effect on the acoustic performance in some cases.